Why is the aspect ratio for this UV-unwrap wrong?
Like i said i'm still a novice.  

The devil was in the details, as a result of the beveling on the top there were still some minor issues (overlapping face, 2 vertices to be joined) at the outer right side of the object. After I took care of those issues the unwrap already looked much better.
Because the front face still showed some stretching, I divided (1 time horizontal and vertical) the front face. Then the unwrap worked like a charm with almost no stretching / deformation.

Took me many days, but i learned a lot !!